If you are a web programmer that designs website for clients then you should always consider what functionality your client will need in order to keep the website updated. Even if your client asks you to do all the updates to the site and does not require access to the back end, it is good to build this in any way in case they change their mind down the line and also to make updates quicker and easier for yourself.

Your client may require access to the back end system of the site to allow them to upload additional content, change or add products etc. Some developers will use an off the shelf content management system such as WordPress or Drupal and others use their own built system. There are pros and cons to using both and often it comes down to what functionality and flexibility you need. If you are using an off the shelf system, then these are usually developed in a fairly straight forward way, but if you have made changes to the templates or themes etc. then you may need to check everything still works as it should.